In recent years, severe requirements have been imposed upon bearing materials for use at high temperatures, such as a heat-resisting carburized steel, in terms of higher temperature and higher speed operating conditions required of rolling bearings, or higher performance required of rolling bearings, which is obtained by adding core toughness and surface compressive residual stress to the rolling bearing materials.
Such a heat-resisting steel is a high alloy steel containing chromium (Cr), nickel (Ni), silicon (Si), molybdenum (Mo), vanadium (V), tungsten (W), etc. For example, there are known AISI M50 NiL as a material for use in engines of jet planes and gas turbines, and CBS 600 and Pyrowear 53 which contain Si in large amounts as materials for enhancing the heat resistance in particular.
In hardening the surface of a high alloy steel containing chromium (Cr), molybdenum (Mo), and silicon (Si) in large amounts such as stainless steel and tool steel, if ordinary gas carburization is employed, a dense oxide film is formed on the surface during carburization, or heavy intergranular oxidation occurs, making it difficult to carburize the steel using the ordinary gas carburization (cf. Journal of Japan Institute of Metals, Vol. 50, No. 4, p395-p402).
To carburize such a high alloy steel, a method has been employed, which includes carrying out pre-oxidation for forming an oxide layer on the surface of the steel in air at 955.degree. C., before carrying out a carburizing treatment, as described in "Specialty Carburizing Steels for Elevated Temperature Service" by C. F. Jatczak; "METAL PROGRESS", p70-p78, April, 1978. The pre-oxidation makes the dense oxide film porous, and accordingly the carburizing gas can contact the surfaces of the resulting bearing (inner ring, outer ring and rolling element).
On the other hand, as a method which does not use pre-oxidation, a plasma carburizing method as disclosed, e.g., in Laid-Open Patent Publication No. 1-225764 and Laid-Open Patent Publication No. 1-255656 can be used to carburize a high alloy steel. This method is widely employed.
Further, an example in which a vacuum carburizing method is applied to stainless steel has been reported (cf. Heat Treatment, Vol, 22, No. 3, p172-p174).
According to the gas carburizing method using the pre-oxidation, however, the chromium (Cr) content is a little bit larger (particularly, if it is 3 weight % or more), the effect of the pre-oxidation becomes reduced, making it difficult to perform carburizing.
Moreover, according to the gas carburization, very small amounts of C.sub.2 O and H.sub.2 O contained in the gas as gas ingredients cause formation of an intergranular oxide layer on the surface of the steel material, particularly if the Si content is large, spoiling the carburizability. Si is an element that enhances the tempering softening resistance, and therefore if the resulting bearing is used in a hot environment, the presence of Si largely affects the function of the bearing.
Besides, if the contents of Cr, Mo, V, etc. of the alloy increase, a dense carbide layer is formed on the steel surface, whereby carbon which is to penetrate and diffuse into the steel base is consumed in the formation of the carbide layer and accordingly the concentration of carbon solid-solved in the steel decreases, so that carbon cannot deeply penetrate and diffuse into the steel base, which requires prolonging the carburizing time. This, however, causes further enlargement of the intergranular oxide layer on the steel surface, leading to degraded strength.
On the other hand, the plasma carburizing method which does not include pre-oxidation does not only require the provision of equipment for carrying out the vacuum carburizing method but also requires power equipment to obtain plasma discharge, which are very expensive, thus leading to an increased treatment cost.
Further, depending upon the combination of the above-mentioned elements, even if these elements are added in amounts which were conventionally not deemed to be critical, for example, if Si and V are added in amounts within a certain range, not only intergranular oxidation occurs but also an oxide reaction with Si and V occurs to cause insufficient local carburization if the gas carburization is employed.
The present inventors have diligently made studies to solve the above described problems, and as a result, have found a range within which carburizing of a high alloy steel which was difficult to perform using the conventional gas carburization can be performed, and a range within which adaptation (implementation) of carburization to a high alloy steel is possible, which was not clear according to the conventional vacuum carburizing method, by limiting ingredients of the alloy steel and adapting a vacuum carburizing method using acetylene, thus successfully solving the problems.
In particular, by employing the vacuum carburizing method using acetylene, the range of possibility of adaptation of the vacuum carburization which was conventionally not made clear has been made clear (in the conventional method, a hydrocarbon-based gas such as propane was used as the carburizing gas), and thus a method of manufacturing a bearing which is inexpensive and has high performance as compared with other methods is provided.
Thus, it is the object of the present invention to provide a method of manufacturing a rolling bearing which is capable of manufacturing a high-performance bearing formed of a heat-resisting carburized steel at a cost lower than that of the conventional method, without degradation in the strength due to formation of the intergranular oxide layer.